This invention relates to an electronic telephone set which comprises first and second connection points for a telephone line, a transmission circuit including a send output stage having a first transistor whose first main electrode is coupled to the first connection point by means of a first resistor and whose second main electrode is coupled to the second connection point, and which set includes a supply circuit for peripheral circuits. This supply circuit has a first output terminal coupled to the first main electrode of the first transistor and has a second transistor whose first main electrode is coupled to the second connection point and whose second main electrode is coupled to the second output terminal of the supply circuit. The circuit includes an operational amplifier whose output is coupled to the gate (control) electrode of the second transistor. The set further includes a first voltage source which is inserted between the first main electrode of the second transistor and a first input of the operational amplifier.
Such an electronic telephone set having a similar supply circuit is known from Philips' Central Application Laboratory Report No. ETT 8707 by F. van Dongen "TEA-1081: A supply IC for Peripheral Circuits in Electronic Telephone Sets", Oct. 1987, Eindhoven, The Netherlands; specifically FIG. A11 on page R 38 of this report. This prior-art supply circuit comprises the commercially available IC TEA1081 and further a capacitor (C.sub.L) and resistor (R.sub.L) as external components providing, in combination with an internal resistor (R.sub.S) of this IC, an inductive input impedance (L=C.sub.L R.sub.L R.sub.S) so that the supply circuit can be connected in parallel with the transmission circuit, which transmission circuit may be a commercially available IC of the TEA1060 family. For a description of this IC family cf. Philips Central Application Laboratory Report by P. J. M. Sijbers "TEA1060 family, Versatile Speech/Transmission ICs for Electronic Telephone Sets", Designers' Guide, Jul. 1987, Eindhoven, The Netherlands.
The prior-art supply circuit has the following features. The d.c. current consumed by the supply circuit is not limited so that when powered with specific current supplied by the telephone line, it may reach such a magnitude that the d.c. current through the first transistor becomes too low to realize the desired send level (microphone signal) and also the stabilizing function of the send output stage of the transmission circuit is lost. Alternatively, the d.c. current through the first transistor of the transmission circuit should be approximately 10 mA, so that with minimum telephone line currents (for example, 15 mA) too little input d.c. current (for example, 4 mA) will remain for the supply circuit. The latter current naturally depends not only on the power taken from the output of the supply circuit but also on the a.c. voltage signal on the telephone line and, consequently, may exceed 4 mA at a specific required output power level.